1. Field of the Invention
This invention relates to a ceramic honeycomb filter for removing floating fine particles from exhaust gas, and more particularly to a small ceramic honeycomb filter having excellent heat resistivity and a large filtering area.
2. Description of the Prior Art
Various conventional filters have been used for removing floating particles from gas; for instance, a filter with filler such as adsorbing material or the like arranged therein, steel wool, ceramic fibers, porous porcelain, and other porous materials. In the case of filters to remove fine particles, means for making the filtering meshes small are used in order to increase removing efficiency thereof, but such means result in a disadvantage that the pressure loss therein becomes large when a high filtration rate is required. Moreover, the structure of the conventional filters is limited to comparatively simple ones, e.g., sheet-formed, plate-formed, or cylindrical filter, and filters of corrugated structure and double cylindrical form are used in order to make their filtering areas large. However, it has been difficult to remarkably increase the filtering areas of the conventional filters. Thus, to avoid the large pressure loss, a filter of large volume is inevitable, and it has been very difficult to make a small filter.
On the other hand, a honeycomb structural body has widely been used as a carrier of catalyst to clean automobile exhaust gas, a heat exchanger of gas turbine or the like. The honeycomb structural body has noticeable advantages in that fluid pressure loss therethrough is low because a multiplicity of parallel channels of certain configuration are uniformly disposed therein, that the surface area per unit volume thereof is large, and that partition walls between the adjacent channels can be made thin to allow easy heat-up thereof. The actual application of the honeycomb structural body, however, has been limited to heat exchangers and catalyst carriers. In the case of the catalyst carrier, suitable catalyst is coated on the thin partition walls of numerous channels, so as to remove noxious gases such as carbon monoxide (CO), hydrocarbons, (HC.sub.n), nitrogen oxides (NOx), etc.
It has been proposed to make a ceramic honeycomb filter by forming a honeycomb structural body having numerous channels with porous ceramic material, sealing selected channels at one ends thereof while sealing the remainder of the channels at the opposite ends thereof in such a manner that dust-containing gas flows therethrough from said one ends to the opposite ends. Since every channel which is open at said one end is closed at the said opposite end, the gas has to pass through the partition walls. Thereby, one can obtain a filter having a large effective filtering area and a low pressure loss.
Accordingly, sealing material used in the proposed ceramic honeycomb filter should tightly adhere to the partition walls, so as to prevent the dust-containing gas from passing through the ceramic honeycomb filter without being filtered at the aforementioned partition walls. In case of using the ceramic honeycomb filter at a high temperature, both the sealing material and the honeycomb structural body must have high heat resistances.
As a sealing material having a high heat resistance and a high adhesiveness, alumina cement has advantages in that it hardens at room temperature due to hydration of calcium aluminate and that it adheres so strongly that a ceramic honeycomb filter can be produced without re-firing after the aforesaid sealing. On the other hand, the alumina cement has a shortcoming in that calcium oxide (CaO) in the alumina cement tends to corrode the partition walls of the ceramic honeycomb structural body at a high temperature, resulting in possible deterioration of heat resistance and filtering ability thereof.